The present invention relates to a method of and an apparatus for continuously measuring the adsorption-desorption isotherms of gases from solids. It also relates to the use of this apparatus particularly in a method of measuring the specific surface area of an adsorbent material.
Two major types of data may be extracted from the adsorption-desorption isotherms of gases from solids: First, in terms of physisorption, that is, at low temperatures, using molecules which do not give rise to specific interactions with particular surfaces sites (for example nitrogen argon or krypton at a temperature of 77.degree. K.), the adsorption-desorption isotherms make it possible to characterize the textural properties of the solids. By processing some or all of these isotherms by well-known theories, e.g., BET, t, BJH, etc. (see, for example, "Adsorption, Surface Area and Porosity", S.J. Gregg and K.S.W. Sing, Academic Press Inc., second edition, 1982), one can determine such values as, for example, specific surface area, microporous volume, and porous distribution. Second, in terms of chemisorption, that is, at high temperatures, using molecules which give rise to a specific interaction with certain particular surface sites (for example ammonia carbon monoxide or hydrogen at temperatures in excess of 273.degree. K.), it is possible to characterize the acidity or the metallic nature of the solids, i.e., the surface functions.
The textural properties and surface functions solids affects a variety of major industrial processes and fields such as, for example, catalysis, adsorption and cement production.
The adsorption-desorption isotherms of gas may be determined intermittently or continuously, the first technique being by far the more used. The intermittent technique consists, for adsorption, in injecting known doses of gas into an enclosure containing the previously pretreated solid. For desorption, the operation is reversed and the known quantities of gas are removed from the enclosure containing the sample. The two branches of the isotherm (adsorption-desorption) are thus described point by point. A plurality of commercial apparatuses which perform this type of procedure are available on the market and include, for example, equipment developed by Micromeritics and Erba Science.
Apparatus based on intermittent injection and extraction of known quantities of gas have a number of drawbacks. This type of procedure is not readily suited to the precise description of isotherms of type I in the low partial pressure range, because the volumes to be injected then become extremely small. On the other hand, the intermittent description of the isotherm does not make it possible to demonstrate the irregularities of minor amplitude which are presented by certain solids and which may reveal rearrangements of the adsorbed phase (see, for example, J. Rouquerol, F. Rouquerol, Y. Grillet and R.J. Ward in "Characterization of porous solids", Studies in Surface Science and Catalysis, Vol. 39 (1988), p. 67.
In principle, the intermittent adsorption or desorption technique is the best. It involves injecting (or withdrawing) the gas at constant and sufficiently low velocities that a thermodynamic equilibrium is constantly maintained. This technique has been proposed for a long time and has been perfected particularly by J. Rouquerol et al. French Patent Application No. 8810972, now French Patent 2,635,383), which demonstrates its feasibility. This technique, as it has been perfected recently, is, however, one which enjoys a limited range of application, a range which depends upon the natural pairing of adsorbate and temperature. For example, for nitrogen at 77.degree. K. the limit partial pressure is around 0.4. This limitation is fairly simple to explain. To obtain low and constant rates of flow, conventional apparatuses employ a loss of head provided by a capillary to which a constant pressure is applied. The flow in such a system is more or less governed by Poiseuille's law which states that the rate of flow (Q) is proportional to the loss of head measured between the inlet to and outlet from the capillary. Therefore, the rate of flow is only approximately constant if the loss of head is itself constant. Well, when the pressure rises in the sample cell, the pressure upstream of the capillary being fixed and being generally less than 8 bars, the loss of head diminishes and consequently the rate of flow likewise diminishes. To ensure a constant rate of flow, one is thus obliged, when using nitrogen at 77.degree. K., to confine oneself to a finally fairly low partial pressure (approximately 0.4). Nevertheless, it is possible to trace complete adsorption isotherms. For example, one can assume a position of 77.degree. K. and employ a gas of which the vapor pressure at this temperature is low such as, for example, argon (vapour pressure at 77.degree. K.=26 KPa). This introduces a considerable stress because the textural properties of the solids are in the majority of cases extracted from measurements based on nitrogen adsorption and it is fairly tricky to back up these measurements with those based on the use of other gases. This is linked to the fact that one knows only with considerable inaccuracy the surface area occupied by a molecule of gas (nitrogen, argon, krypton) and an offset of 10 to 20% between the nitrogen and argon measurements is normal (see, for example: "Adsorption, Surface Area and Porosity", S.J. Gregg and K.S.W. Sing, Academic Press Inc., second edition, 1982). Another method relates to mass control (U.S. Pat. No. 4,489,593). The range of flow rates necessary to establish a thermodynamic equilibrium being beyond the scope of conventional flow meters (flow rates generally less than 0.7 ml/min), it is therefore necessary to use flow meters which are especially adapted (U.S. Pat. No. 4,489,593) to the range: 0.05 to 0.7 ml/min. The apparatus constructed around these mass flow meters is complex and has recourse to a sophisticated electronics unit, so that the flow meters have to be supplied with an entirely constant pressure and placed in a thermostatically controlled enclosure. Indeed, a 1.degree. C. fluctuation in the temperature of the gas introduced into the flow meters becomes translated into a fluctuation in rate of flow of about 1.5%, which is considerable. In practice, the apparatus described in U.S. Pat. No. 4,489,593 does not use a pure gas but a mixture for adsorption. Thus, for measurement of the isotherm of nitrogen adsorption at 77.degree. K., a mixture of nitrogen and helium is used, which makes it necessary to use a reference ampoule so that the system is singularly complicated. One of the objects of the invention is to remedy the above-mentioned disadvantages.
Another object of the invention is to supply at any time and into the measuring circuit a substantially constant rate of gas flow such that the pressure of this gas in the measuring circuit containing the adsorbent sample is substantially in equilibrium.